"All 54 survivors have been rescued and are now aboard," Mediterranea, which is made up of left-wing activists, tweeted.

"Among them 11 women (three pregnant) and 4 children.... Happy to have rescued 54 lives from the hell of Libya. Now we need a safe port."

Italy's far-right Interior Minister Matteo Salvini who has tried to close Italian ports to migrant rescue vessels swiftly tweeted that Mediterranea's 18-metre (59-foot) sailboat Alex should take those rescued to Tunisia.

"Otherwise we will activate all procedures to prevent Italy being a port of arrival for human traffickers," Salvini said.

Italian authorities last week seized a vessel belonging to German aid group Sea-Watch and arrested its captain after it entered Lampedusa port in Sicily unauthorised with dozens of rescued migrants on board.

Mediterranea said earlier that the Alex was heading for a rubber boat in distress in Libya's search and rescue area.

Italian authorities informed them that the area was the responsibility of the Libyan coast guard which was sending vessels, Mediterranea said.

"But people must be rescued, not captured and deported back to Libya."

"Libyan patrol boat arrived late, they first asked us to stop, then left the scene," the collective tweeted.

Another makeshift vessel carrying 86 people across the Mediterranean from Libya sank off the Tunisian coast on Wednesday, with all but three of them believed drowned.

Libya, which has been wracked by chaos since the 2011 uprising that killed veteran dictator Moamer Kadhafi, has long been a major transit route for migrants, especially from sub-Saharan Africa, desperate to reach Europe.

On Tuesday night, 44 migrants were killed in an air strike on their detention centre in a Tripoli suburb.